Add Jupyter server extension to serve files

[philippjfr]

In a Bokeh server context we serve the panel/dist directory on an endpoint, which makes it possible to load all bundled resources from there, while in the notebook we have to rely on either inlining the resources or loading it from CDN. This PR adds a server extension to Panel, which serves the panel/dist directory under a /panel_dist/ endpoint in Jupyter.

[codecov]

Codecov Report

Merging #1982 (d428554) into master (612ca40) will decrease coverage by 0.03%.
The diff coverage is 27.27%.

@@            Coverage Diff             @@
##           master    #1982      +/-   ##
==========================================
- Coverage   84.87%   84.84%   -0.04%     
==========================================
  Files         157      158       +1     
  Lines       19360    19369       +9     
==========================================
+ Hits        16432    16433       +1     
- Misses       2928     2936       +8     

Impacted Files	Coverage Δ
panel/io/jupyter_server_extension.py	0.00% <0.00%> (ø)
setup.py	0.00% <ø> (ø)
panel/io/notebook.py	57.29% <50.00%> (-0.08%)	⬇️
panel/__init__.py	100.00% <100.00%> (ø)
panel/io/__init__.py	100.00% <100.00%> (ø)

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